We aim to understand how the various components of visual cortical circuits contribute to the computations that give rise to visual perception. To this end, we have done detailed anatomical and physiological studies of local connectivity in primary visual cortex. These studes have recealed that excitatory connections can be classified as one of two types: those that are strong or driving and those that are weak or modulatory. These properties appear to be correlated with the type of pyramidal neuron providing the synaptic 9input. (We have previously identified approx 10 types of neurons in deep layers of primate primary visual cortex by intracellular labeling and light microscopic analysis.) Studies in other labs have indicated that the functional influence of a synaptic input is strongly correlated with ultrastructural properties of the synapse, such as the sizes of pre- and postsynaptic densities, and the numbers of docked synaptic vesicles. We therefore propose to mea sure these parameters for synapses that are provided by cells of known type. Individual neurons will be intracellularly labeled in living brain slices and characterized anatomically at the light level for classification. Synapses that are labeled presynaptically will then be reconstructed with the high-voltage EM to identify morphological characteristics of synapses arising from the different cell types. We have begun work on this project to determine the optimal method for labeling neurons, including fixation, permeablization and intensity of diaminobenzidine labeling. We have performed one serial section reconstruction and anticipate performing tomographic reconstructions shortly.